1. Field of the Invention
The present invention relates to an improved electrode used as a standard electrode, also referred to as a reference electrode, standard electrode, assistant electrode and the like, for a measuring electrode (for example, a glass electrode) in the case where an ionic concentration (for example, pH) of a solution is measured.
2. Description of the Prior Art
A reference electrode is indispensable in the measurement of various kinds of ionic concentration and it is important that an inter-liquid potential difference that can show a small change be stabilized, regardless of the kind and concentration of a sample solution to be tested.
In a conventional reference electrode, as shown in FIG. 10, an internal electrode formed of, for example, a Ag electrode wire b, having a surface coated with AgCl, and an internal solution d formed of a 3.3 M-aqueous solution of KCl supersaturated with AgCl with a phosphoric acid buffer solution added are provided on a support pipe a formed of an electrically insulating glass. In order to construct a liquid junction, the internal solution d is brought into contact with the sample solution through a liquid junction member e formed of an inorganic sintered porous material, an organic high molecular porous material and the like, impregnated with, for example, KCl and positioned in a hole formed in a pointed end portion of the support pipe a. Referring to FIG. 10, reference mark f designates an internal solution make-up port provided with a cover g mounted on the support pipe a for permitting a make-up of any internal solution d that has been consumed.
That is to say, this conventional reference electrode provides a single pole cell schematically expressed by Ag/AgCl/3.3M-KCl/with a Liquid junction member (Sample solution). A potential E.sub.o generated in the Ag/AgCl/3.3 M-KCl cell itself should be completely compensated, so that it will not appear in the measurement result, for example, by using the same one Ag/AgCl/3.3 M-KCl cell also in an internal electrode on a side of a measuring electrode (for example, a glass electrode). However, since the 3.3-M aqueous solution of KCl, which is the internal solution d, is diffused into the sample solution in the form of K.sup.+ and Cl.sup.- through the liquid junction member e, an inter-liquid differential potential (referred to also as a differential diffusion potential) E.sub.D, which cannot be compensated to some extent, cannot be avoided.
This inter-liquid differential potential ED can be expressed by the following equation: ##EQU1## wherein T.sup.+ : Transport coefficient of a cation n the internal solution d;
T.sup.- : Transport coefficient of an anion in the internal solution d; PA1 R: Ideal gas constant; PA1 T: Absolute temperature; PA1 F: Faraday constant; PA1 a.sub.D : Activity of the sample solution; and PA1 a.sub.kc : Activity of the internal solution.
Accordingly, in order to reduce and stabilize this inter-liquid differential potential E.sub.D, it would be ideal if an internal solution d having a large limiting equivalent ionic conductivity and a transfer coefficient T.sup.+ of a cation and a transfer coefficient T.sup.- of an anion equal to each other is used, and also that the concentration of the internal solution d be increased. This is one of the reasons why an aqueous solution of KCl of high concentration has been generally used as the internal solution d of the reference electrode.
Tables 1 and 2 showing a limiting equivalent ionic conductivity of main ions in an aqueous solution of 25.degree. C., a transport coefficient of main electrolytes and a solubility of main electrolytes in an aqueous solution of 25.degree. C., both the limiting equivalent ionic conductivity (73.5 cm .sup.-1) of a cation K.sup.+ constructed from KCl and the limiting equivalent ionic conductivity (76.35 cm .sup.-1) of an anion C1.sup.-, also constructed from KCl, are comparatively large and nearly equal to each other. In addition, the transport coefficient T.sup.+ (0.490) of a cation of KCl is nearly equal to the transport coefficient T.sup.- (0.509) of an anion of KCl.
Although crystalline KCl in theory could be satisfactorily used, KCl is an insulator and, therefore, a saturated aqueous solution of KCl having a high concentration has been frequently used. In addition, in the case where the sample solution contains a substance acting upon the aqueous solution of KCl, a disadvantage occurs in that gases are generated or compounds are deposited to destabilize the interliquid differential potential E.sub.D or clog the liquid junction member e, so that in such a case an aqueous solution of KNO.sub.3 is used as the internal solution in place of the aqueous solution of KCl or an aqueous solution of NH.sub.4 NO.sub.3 is used as an overcoat solution.
TABLE 1 ______________________________________ [Limiting equivalent ionic conductivity (cm .OMEGA..sup.-1)] Cations Anions ______________________________________ H.sup.+ 349.8 OH.sup.- 198.3 Rb.sup.+ 77.8 Br.sup.- 78.14 K.sup.+ 73.5 Cl.sup.- 76.35 Ag.sup.+ 61.9 NO.sub.3.sup.- 71.5 Ca.sup.+ 59.5 CO.sub.3.sup.- 69.3 Mg.sup.+ 53.0 F.sup.- 55.4 Na.sup.+ 50.1 Li.sup.+ 38.6 ______________________________________
TABLE 2 ______________________________________ [Transport coefficient of ions and solubility] Transport Transport coefficient coefficient Solubility Electrolyte T.sup.+ of cation T.sup.- of anion (mol/l) ______________________________________ KCl 0.490 0.509 26.4 MgF.sub.2 0.489 0.510 8.4 .times. 10.sup.-3 CaF.sub.2 0.517 0.482 1.6 .times. 10.sup.-3 ______________________________________
The above described conventional reference electrode has been used with an aqueous solution of KCl, as an internal solution d, which is diffused into a sample solution through a liquid junction member e formed of porous material. Since the aqueous solution of KCl is brought into contact with the sample solution and in order to prevent the contamination of the aqueous solution of KCl with the sample solution, the aqueous solution of KCl is positively leaked in the sample solution so that in the case where such a reference electrode is used, it is necessary to pay close attention to the following matters:
(A) The aqueous solution of KCl is maintained at an appointed constant concentration as far as possible;
(B) The aqueous solution of KCl is adapted to leak into a side of the sample solution (the sample solution is prevented from being mixed in the aqueous solution of KCl) by slightly heightening a pressure of the aqueous solution of KCl;
(C) Attention must be paid to the consumption of the aqueous solution of KCl and it is replenished if necessary;
(D) Attention must be paid to prevent any clogging of the liquid junction member e (a usual inter-liquid impedance amounts to several hundreds .OMEGA. to ten and several k.OMEGA.) and the like.
Accordingly, a reference electrode having the above-described construction has exhibited many disadvantages in operation, maintenance and the like. In addition, even though sufficient attention could be paid to such matters, the generation of the inter-liquid differential potential E.sub.D cannot be avoided, so that it has been very difficult to achieve a highly accurate measurement.